1. Field of the Invention
This invention relates generally to laser marking techniques and, more specifically, to an apparatus and method for marking the surface of a bare or packaged semiconductor device comprising one or more dice, using a laser and a laser reactive material.
2. State of the Art
Since the first semiconductor devices became commercially available, manufacturers have found it necessary to mark each chip or assembly of chips (bare die or package) with the company name, a part or serial number, or other information such as lot number or die location. Conventional marking methods utilize a mechanical device to transfer ink contained in an ink pad to the surface of a stamp. An individual chip is then stamped, and the automated process is repeated for subsequent chips.
Because of its mechanical nature and the drying time associated with ink, an ink stamping process is relatively slow. Moreover, if the mark is accidentally touched prior to complete drying, the mark will smudge. In chip manufacturing processes using such an ink stamping method, the ink marking operation may have to be included at a relatively early stage of production (if the die itself is to be marked) or just after post-encapsulation processing (if the package is to be marked) to allow for drying time without affecting the production rate. Such early marking may result, however, in marking defective chips that never make it completely through the manufacturing process.
Another problem associated with ink stamping methods is that the quality of ink stamped marks may substantially vary over time. This variation may be dependent upon the quantity of ink applied, ambient temperature and humidity, and/or the condition of the surface of the stamp. In any event, the consistency of a stamped mark may vary widely from chip to chip.
As a result of the deficiencies associated with ink stamping, it has become increasingly popular to use a laser beam to mark the surface of a chip. Unlike ink stamping, laser marking is very fast, requires no curing time, has a consistently high quality, and can take place at the end of the manufacturing process so that only good chips are marked.
Various machines and methods have been developed for marking a chip with a laser. As illustrated in U.S. Pat. No. 5,357,077 to Tsuruta, U.S. Pat. No. 5,329,090 to Woelki et al., U.S. Pat. No. 4,945,204 to Nakamura et al., U.S. Pat. No. 4,638,144 to Latta, Jr., U.S. Pat. No. 4,585,931 to Duncan et al., U.S. Pat. No. 4,375,025 to Carlson, a semiconductor device is placed in a position where a laser beam, usually produced by a carbon dioxide, Nd:YAG, or Nd:YLF laser, inscribes various characters or other information on a surface of the semiconductor device. Basically, the laser beam burns the surface of the chip such that a different reflectivity from the rest of the chip surface is formed. By holding the chip at a proper angle to a light source, the information inscribed on the chip by the laser can be read. Various materials are known in the art that are laser reactive (e.g., capable of changing color when contacted by a laser beam). As described in U.S. Pat. Nos. 4,861,620 to Azuma et al., U.S. Pat. No. 4,753,863 to Spanjer, and U.S. Pat. No. 4,707,722 to Folk et al., the part or component may be partially comprised of the laser markable material or have a coating of the material on the surface of the part or component to be marked.
Using a laser to mark a chip is a fast and economical means of marking. There are, however, certain disadvantages associated with state-of-the-art laser marking techniques that merely burn the surface to achieve the desired mark in comparison to ink stamping. For example, ink stamping provides a clearly visible image on the surface of a chip at nearly every angle of incidence to a light source. A mark burned in a surface by a laser, on the other hand, may only be visible at select angles of incidence to a light source. Further, oils or other contaminants deposited on the chip surface subsequent to marking may blur or even obscure the mark. Additionally, because the laser actually burns the surface of the work piece, for bare die marking, the associated burning may damage the internal circuitry of the chip directly or by increasing internal die temperature beyond acceptable limits. Moreover, where the manufactured part is not produced of a laser reactive material, laser reactive coatings applied to the surface of a component may take hours to cure.
Thus, it would be advantageous to provide a marking technique that combines the speed and precision of laser marking with the contrast and distinctiveness of ink stamping, without any substantial curing or drying time. Moreover, it would be advantageous to develop a method and apparatus for marking the surface of a semiconductor chip that does not harm the circuitry enclosed therein.
According to the present invention, a laser marking apparatus and method are disclosed wherein an object is subjected to a laser beam or other suitable energy source for marking purposes. While the laser beam is actively marking, a substance is introduced into the marking work area that interacts with the laser beam. The substance reacts with the localized heat created by the laser and forms a new compound on the surface of the package or surface of the chip. This new compound is selected to contrast highly with the color and/or surface texture of the surface that has been marked.
In another particular aspect of the invention, the surface of a chip is at least partially covered with a laser reactive substance prior to being contacted by a laser beam. The substance may be in either liquid or powder form and may be rolled on, sprayed on, or otherwise applied by means known in the art. When subjected to the localized heat created by the laser, a semi-permanent, solvent-removable mark is formed and bonded to the surface of the chip. The excess material on the non-irradiated portion, that is, the portion of the surface not contacted by the laser beam, is readily removed by an exhaust or residue removal system and may be recycled for future marking.
In another, more particular aspect of the invention, an ink bearing material, or other pigmented or laser reactive substance-bearing material is disposed adjacent to an exposed surface of a chip. The laser beam transfers ink contained in the ink bearing material to the exposed surface of the chip. For example, the ink bearing material may comprise a ribbon contained in a ribbon dispenser. During the marking process, as the laser beam transfers ink from one point on the ribbon to the chip, another segment of the ribbon may be exposed to the laser beam for subsequent markings. Such an ink bearing material may also help to reduce heat produced by the laser beam from substantially penetrating the surface of the marked chip.
In a more particular aspect of the invention, a stream of atomized particles of B-stage epoxy with an added pigment of a desired color (white for example) is directed at the surface where the laser is actively marking the specimen. The epoxy reacts to the heat of the laser and cures to a visible white image coincident with the path of the laser. The excess particles, those which have not been directly irradiated by the laser beam, may be removed along with other debris from the work area by a debris removal system.
In another, more particular aspect of the invention, much of the epoxy is destroyed by the laser. A thermal gradient, however, along the trailing edge of the laser path causes the epoxy to cure normally into a final and permanent state, thus producing the desired mark.
In another particular aspect of the invention, the laser reactive material absorbs most of the heat produced by the laser. As a result, the delicate internal circuitry of the chip is not exposed to this potentially damaging heat.
In another aspect of the invention, subsequent to or while being marked, the chip is subjected to a jet of coolant to rapidly cool the markings and prevent or reduce the potential for heat damage to the chip. The coolant may be in a liquid, gas, or solid state. In this manner, any residual heat contained in the marking material or present in the surface of the chip may be rapidly dissipated. The markings are thus completely cured and/or cool before exiting the marking apparatus.
In another, more particular aspect of the invention, the laser marking apparatus is computer controlled. In addition to controlling the laser beam, chip location, and other process parameters, the central processing unit (CPU) may control the quality of markings. If so, the marked chips may be subjected to a camera which feeds an image of each chip to the CPU. The CPU compares the pixels of the captured image to a given resolution standard. If the marking is of a sufficiently high quality, the chips are automatically accepted. If not, the chips are automatically rejected for rework and remarking.